1. Field of the Invention
This invention relates generally to radio frequency interference impairments associated with digital subscriber lines, and more particularly to a method for exploiting deterministic spreading of the fast Fourier transform (FFT) at the receiver to cancel correlated noise sources such as RFI.
2. Description of the Prior Art
ITU Telecommunications Standardization Sector, G.test.bis., Study Group 4, Question 15, 1999, has proposed various radio frequency interference (RFI) impairments for incorporation into digital subscriber line (DSL) performance tests. For asymmetric DSL (ADSL) systems such as described in ITU Telecommunications Standardization Sector, G. 992.1: Asymmetrical Digital Subscriber Line (ADSL) Transceivers., Study Group 4, Question 15, 2000, RFI is assumed to be caused by narrowband amplitude modulated (AM) radio stations, which overlap the downstream ADSL band from 540–1104 kHz. Additional RFI sources include, but are not limited to, aliases from AM stations operating above 1104 kHz, amateur (HAM) transmitters, televisions, computer equipment, and various other (relatively) low frequency signals (e.g., aeronautical radiolocation signals).
At the ADSL transmitter, data is assembled into subchannels, transformed by an inverse fast Fourier transform (IFFT), and a cyclic prefix is added. After propagating through a communication channel, the opposite operations are performed by the receiver. The cyclic prefix is removed, and the received data is transformed by the fast Fourier transform (FFT). If the communication channel is shorter than the cyclic prefix and the noise is uncorrelated on the individual subchannels, then the transmitted data can be recovered by a one-tap equalizer (the frequency-domain equalizer or FEQ), such as described by T. Starr, J. Cioffi and P. Silverman, Understanding Digital Subscriber Line Technology, Upper Saddle River, N.J.: Prentice Hall, 1999.
The noise that results from RFI at the receiver is deterministically spread by the FFT to neighboring subchannels. As a result, instead of one (or a few) subchannels being effected by the narrowband impairment, many subchannels are effected. The resulting loss of signal-to-noise ratio (SNR) can unacceptably limit the capacity of the communication channel.
In view of the foregoing, a need exists for a scheme to exploit the deterministic spreading of the FFT at the receiver to cancel correlated noise sources such as RFI in order to increase the effective data communication rate between subscribers using shared media.